1. Field of the Invention
The present invention relates to an antenna for a backscatter-based RFID (radio frequency identification) transponder, and a backscatter-based RFID transponder having such an antenna.
2. Description of the Background Art
The invention resides in the field of wireless and contactless communication. It resides particularly in the field of radio-based communication for the purpose of identifying objects, animals, persons, etc., as well as the transponders and remote sensors used for this purpose.
While applicable in principle to any desired contactless communication system, the present invention and the problem on which it is based are described below with reference to RFID communications systems and their applications. In this connection, RFID stands for “Radio Frequency Identification.”
In RFID systems, data is transmitted bidirectionally with the aid of high-frequency radio signals between a stationary or mobile base station, which is often also referred to as a reader or read/write device, and one or more transponders that are attached to the objects, animals or persons to be identified.
The transponder, which is also referred to as a tag or label, typically has an antenna for receiving the radio signal emitted by the base station, as well as an integrated circuit (IC) connected to the antenna. In this context, the integrated circuit includes a receive circuit for receiving and demodulating the radio signal and for detecting and processing the transmitted data. In addition, the integrated circuit has a memory for storing the data needed for identification of the corresponding object. Furthermore, the transponder can include a sensor, for example for temperature measurement, which is likewise part of the integrated circuit, for instance. Such transponders are also known as remote sensors.
RFID transponders can be used to advantage anywhere that automatic identification, detection, interrogation, or monitoring is to take place. The use of such transponders makes it possible for objects such as, for example, containers, pallets, vehicles, machines, or pieces of luggage, but also animals or people, to be individually marked and identified in a contactless way without a line-of-sight connection. In the case of remote sensors, it is additionally possible for physical qualities or parameters to be measured and interrogated.
In the area of logistics, containers, pallets and the like can be identified, for example in order to determine their current whereabouts during the course of shipping. In the case of remote sensors, the temperature of the transported goods or products can be regularly measured and stored, for example, and read out at a later point in time. In the area of cloning protection, items such as integrated circuits can be provided with a transponder in order to prevent unauthorized reproduction. In commercial applications, RFID transponders can replace the barcodes often placed on products. Additional applications include, for example, driveaway protection in the automotive field, or systems for monitoring the air pressure in tires, as well as in systems for personal access control.
Passive transponders have no independent energy supply, and extract the energy required for their operation from the electromagnetic field emitted by the base station. Semi-passive transponders, while they do indeed have their own energy supply, do not use the energy provided by it to transmit/receive data, but instead use it to operate a sensor, for example.
RFID systems with passive and/or semi-passive transponders whose maximum distance from the base station is significantly over one meter are operated in particular in frequency ranges in the UHF or microwave range.
In such passive/semi-passive RFID systems with a relatively long range, a backscattering-based method is generally used for data transmission from a transponder to the base station, in the course of which a portion of the energy from the base station arriving at the transponder is reflected (backscattered). In this process, the carrier signal is modulated in the integrated circuit according to the data to be transmitted to the base station and is reflected by means of the transponder antenna. Such transponders are referred to as backscatter-based transponders.
In order to achieve the greatest possible range with backscatter-based transponders, it is necessary to deliver the largest possible fraction of the energy arriving at the transponder from the base station to the integrated receive circuit of the transponder. Power losses of every type must be avoided in this process. On the one hand, this requires transponder antennas with a relatively broad receive frequency range. Such relatively wide-band antennas can have the additional advantage of meeting the requirements of multiple national or regional authorities with only one antenna type. On the other hand, the energy picked up by the transponder antenna must be delivered, with as little reduction as possible, to the integrated receive circuit, which typically has a capacitive input impedance, i.e. an impedance with a negative imaginary part.
Known from DE 103 93 263 T5, which corresponds to U.S. Pat. No. 6,963,317, is an antenna for an RFID system which has a planar helix structure with two branches. Starting from a central region, each of the two branches extends outward in a helix. The input impedance of this antenna is also capacitive.
A disadvantage here is that the impedance of this antenna differs sharply from the complex conjugate value of the impedance of the chip input circuit, and thus that an additional, separate matching circuit with a coil and a capacitor is required. Because of parasitic resistances of these components, power losses arise in the transponder, disadvantageously reducing the range. Moreover, the separate matching circuit restricts the freedom in placement of the chip and results in more complex and thus more expensive implementations of the transponder.
From the article, “Broadband RFID tag antenna with quasi-isotropic radiation pattern,” by C. Cho, H. Choo and 1. Park, published in Electronics Letters, Vol. 41, No. 20, Sep. 29, 2005, pages 1091-1092, an antenna is known for a UHF RFID system that has two folded dipoles and a twin-T matching network. The area required by this antenna is 79 mm×53 mm. A region of 1.7 m to 2.4 m is given as the range of the RFID system.
However, for many applications only a relatively small area is available. In addition, elongated antennas having a relatively small width of approximately 35 mm and a length of up to 100 mm are advantageous for some applications, and for simple manufacture of the antenna on a strip. Moreover, many applications require greater range.